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1 Department of Chemistry and Physics,
The object of this study is to quantify the very low frequency
(i.e., <0.1 Hz) interactions between renal sympathetic nerve activity
(SNA) and arterial blood pressure (ABP). Six rats were instrumented for
chronic recordings of SNA and ABP. Data were collected 24 h after
surgery at 10 kHz for 2-5 h and subsequently compressed to a 1-kHz
signal. The power spectra and ordinary coherence were calculated from
data epochs up to 1 h in length. The very low frequency spectra for
both variables were fitted to a constant times
f 
. The peak magnitude
squared of the coherence near 0.4 Hz was 0.82 ± 0.08, but the
apparent linear coherence fell off quickly at lower frequencies so that
it was close to zero for frequencies <0.1 Hz. Moreover, at these low
frequencies 
, as computed by a coarse grain spectral analysis, was
significantly (P < 0.01) different for SNA
(0.66 ± 0.12) and ABP (1.12 ± 0.14). Assuming that SNA and ABP
are stationary time series, the results of our classical spectral
analysis would indicate that SNA and ABP are not linearly correlated at
frequencies with a period more than ~10 s. Accordingly, we tested for
stationarity by computing the spectral coherence and found that SNA and
ABP are not stationary "1/f noise" within the frequency
range from 0.02 to 2.0 Hz. Rather the SNA exerts control over the
cardiovascular system through intermittent bursts of activity. Such
intermittent behavior can be modeled by nonlinear dynamics.
coherence; spectral coherence; nonlinearity; blood pressure
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